The Src Homology 2 (SH2) and Src Homology 3 (SH3) domain-containing protein Crk is the prototypical member of a class of adaptor proteins that play essential roles in signaling downstream of tyrosine kinases. By promoting the assembly of protein complexes mediated by the SH2 and SH3 domains, evidence accumulated over the past two decades has elucidated a canonical pathway for Crk signaling whereby the SH2 domain binds tyrosine phosphorylated proteins and the N-terminal SH3 domain (SH3N) binds guanine-nucleotide exchange factors that activate Rac1, Rap1, and Ras. The clinical significance of Crk in human cancer has been enumerated in recent years, as Crk is frequently over-expressed in several different cancers, including breast, ovarian, gastric, lung, glioblastoma, and sarcomas and siRNA-mediated knockdown of Crk reverses the malignant and metastatic features of these cancers. These observations have led to a new urgency to understand the mechanisms by which Crk promotes malignant transformation in the hope that new information can be exploited to develop therapeutics, particularly for tumors with a predisposition towards invasion and metastasis. In this application we have identified a new signaling paradigm for Crk by the identification of two previously uncharacterized tyrosine phosphorylation sites located within the carboxyl-terminal SH3 (SH3C) domain, an atypical SH3 domain that has no clear biological function. Tyr251 is located in the highly conserved RT-loop of the Crk SH3C while Tyr239 is located at the boundary of the linker and SH3C and comprises a region of Crk implicated in the negative regulation and auto-clamping of the SH3C to the SH3N. The central hypothesis of this application is that Crk, in addition to its canonical role as an adaptor protein, has an unconventional role in signal transduction through phosphorylation of Tyr251 and Tyr239, that define new binding sites allowing recruitment of SH2 and/or PTB domain containing proteins. The recruitment of these proteins may initiate post-phosphorylation-dependent signaling pathways relevant to cell transformation. Both sites are tyrosine phosphorylated by EGFR and ErbB2 receptors suggesting this research is directly relevant to signaling in cancer cells that have overexpressed or activated receptor tyrosine kinases oncogenes. In recent years, an emerging body of evidence suggests that Crk proteins are overexpressed in human tumors and the expression levels correlate with aggressive and malignant behavior of cancer cells. These properties of Crk proteins make them potential cancer prognosis markers and therapeutic targets.